An improved treatment of empirical dispersion and a many-body energy decomposition scheme for the explicit polarization plus symmetry-adapted perturbation theory (XSAPT) method

We recently introduced a low-cost quantum chemistry method for computing intermolecular interactions, combining a monomer-based self-consistent field calculation (the explicit polarization method, XPol) with pairwise-additive symmetry adapted perturbation theory (SAPT). The method uses Kohn-Sham (KS) orbitals in the SAPT formalism but replaces the SAPT dispersion and exchange-dispersion terms with empirical potentials (+D), and we called this method XPol+SAPT(KS)+D. Here, we report a second-generation version of this approach, XPol+SAPT(KS)+D2 or XSAPT(KS)+D2 for short, in which we have modified the form of the empirical atom-atom dispersion potentials. Accurate binding energies are obtained for benchmark databases of dimer binding energies, and potential energy curves are captured accurately for a variety of challenging systems. We suggest that using different asymptotic corrections for different monomers is necessary to get good binding energies in general, especially for hydrogen-bonded complexes. As compared to our original +D formulation, the second-generation +D2 method accurately reproduces not only total binding energies but also the various components of the interaction energy, and on this basis we introduce an energy decomposition scheme that extends traditional SAPT energy decomposition to systems containing more than two monomers. For (H2O)6, the many-body contribution to the interaction energy agrees well with that obtained from traditional Kitaura-Morokuma energy decomposition analysis in a large basis set. (C) 2013 AIP Publishing LLC.